Atmospheric boiler efficiency

ethicalpaul

Hey everyone! I see people talking about the efficiency of atmospheric boilers (I guess both hot water and steam?) where they say things like "the efficiency is going to be low until longer into a cycle when the flue warms up"

But how can the flue temperature affect efficiency? Isn't it true that the only measure of efficiency can be between the burner and the heat transfer to the water?

It seems to me that any situation after the boiler cannot affect efficiency at all.

Is it because the combustion analyzer tools take their samples from the flue and so the flue has some "imaginary" role to play because that's where the measurements are? Or am I missing something here?

It even seems to me that efficiency would be higher early in the heating cycle because more heat would transfer when the boiler is cooler than when it is warmer. The closer the burner/combustion gas temperature is to the boiler/heat exchanger, the less transfer will occur, and the more heat will go up the flue instead of into the boiler. Am I wrong there too?

Thanks!!

hot_rod

Im not sure that efficiency is low at start up, the colder the boiler runs the more efficient it runs.
, i agree with that observation.  Viessmann has data on that in their trainings

There are a number of efficiencies to consider.

Combustion, cycle efficiency, distribution efficiency 

EBEBRATT-Ed

Do a combustion test. If all the other readings stay the same the efficiency is higher when the stack temp is lower.

Many on here are adamant about not down firing an oil boiler. Did it for years with the old coaf fired units. A couple of bricks in the flue pipe to slow the draft, cut the size of the combustion chamber or line it. Use a smaller nozzle and up the oil pressure. There is no magic to it. You could increase the combustion efficiency in some cases by 10%. That's not seasonal efficiency you still have large flue passages and a large standby loss. but you could make a substantial improvement.

With todays oil boilers most are overfired for how much heat the sections can absorb. Peerless is one mfg that lists multiple firing rates most of the others don't

I like to pick an oil boiler that I can heat the load with when I fire it at 80-90% of the boiler rating. Better efficiency . stays cleaner and lasts longer.

Gas atmospheric is a different story. Can't do tooo much with those. I have dropped the manifold gas pressure to 3" wc on natural gas and usually have no issues.

Steamhead

@EBEBRATT-Ed makes some good points. I'll add that we must also consider seasonal efficiency.

The best thing one can do with an atmospheric that doesn't have a stack damper, is to put one in. There are two reasons for this:

1- the damper keeps the chimney draft from cooling the boiler down between firing periods, which reduces the heating-up time on the next firing cycle;

2- eliminating this off-cycle draft reduces air infiltration into the building when the burner is off. Less heat escapes the building this way.

Now, someone is going to come on here hating on stack dampers because they have been troublesome in the past and expensive to replace. True, some have been. The current Effikal and Field dampers- basically the same unit with different names- have done much better. Also, the motors on these units, which also contain the safety switches, are replaceable. The replacement takes an hour or less- two screws and a plug for the cable- and costs far less than a completely new unit.

We've run into a lot of old Flair, Johnson and Honeywell dampers that have failed. We replace them with current models when we can.

SuperTech

@Steamhead I agree with everything you stated except the problems persist with the flue dampers. It seems like every time my company installs an atmospheric boiler and leaves the flue damper operational it always results in a service call as a result in 2 or 3 years.  I generally love Field controls products but not those flue dampers. And I'm no fan of the L8148E aquastats that seem to accompany them.

ethicalpaul

2 or 3 years is a good time to check the boiler's operation anyway, no?

Steamhead

SuperTech said:

@Steamhead I agree with everything you stated except the problems persist with the flue dampers. It seems like every time my company installs an atmospheric boiler and leaves the flue damper operational it always results in a service call as a result in 2 or 3 years.  I generally love Field controls products but not those flue dampers. And I'm no fan of the L8148E aquastats that seem to accompany them.

We've had good results with them. Maybe the aquastats are part of the problem?

DJD775

EBEBRATT-Ed said:

Do a combustion test. If all the other readings stay the same the efficiency is higher when the stack temp is lower.

Many on here are adamant about not down firing an oil boiler. Did it for years with the old coaf fired units. A couple of bricks in the flue pipe to slow the draft, cut the size of the combustion chamber or line it. Use a smaller nozzle and up the oil pressure. There is no magic to it. You could increase the combustion efficiency in some cases by 10%. That's not seasonal efficiency you still have large flue passages and a large standby loss. but you could make a substantial improvement.

With todays oil boilers most are overfired for how much heat the sections can absorb. Peerless is one mfg that lists multiple firing rates most of the others don't

I like to pick an oil boiler that I can heat the load with when I fire it at 80-90% of the boiler rating. Better efficiency . stays cleaner and lasts longer.

Gas atmospheric is a different story. Can't do tooo much with those. I have dropped the manifold gas pressure to 3" wc on natural gas and usually have no issues.

@EBEBRATT-Ed This leads me to a question I have been pondering recently. How does a combustion analyzer calculate efficiency between nozzles when it doesn't know how many BTUs are input in the system? If all the combustion numbers stay the same and you have lowered the number of BTUs input into the system then I would assume the stack temperature would be lower but does that mean does that mean the system is more efficient? Since the input rate is not factored into the combustion analyzer I would assume the efficiency reading would only be valid when comparing adjustments when using the same nozzle. Am I missing something or are any of my assumption wrong?

EBEBRATT-Ed

The boiler of any given size can only absorb so much heat. The boiler MFGs are all in competition to compete with each other on a price point. Companies like HB Smith sized their boilers a little more conservatively that is why they are basically out of business.

Boiler mfgs push their output up to use a smaller boiler to be competitive. If one boiler mfg. uses a 3-section boiler and one uses a 4-section boiler to get the same heat output the 3-section boiler is likely less $$$. Longer life better efficiency are obtained by reducing the btu input and output. JMHO

If you fire the boiler with less input the stack temperature must come down. If the other #s stay about the same your efficiency has to increase. Combustion analyzers don't know BTU input, but they do measure stack temp.

Now if you under fire the boiler too much thing will go in the wrong direction. Flue temps drop too low and condensation results. Warm up time gets longer and burns mor fuel.

I am talking a 5-15% decrease in the maximum firing rate.

The Steam Whisperer

SuperTech said:

@Steamhead I agree with everything you stated except the problems persist with the flue dampers. It seems like every time my company installs an atmospheric boiler and leaves the flue damper operational it always results in a service call as a result in 2 or 3 years.  I generally love Field controls products but not those flue dampers. And I'm no fan of the L8148E aquastats that seem to accompany them.

I"ve never had problems with stack dampers failing very often. I think most of them fail due to short cycling because the boiler is oversized, the heat anticipator is set incorrectly or the operating controls (like tekmar) are not set up properly. I've seen plenty of boilers cycling every 10 minutes instead of once or twice an hours... that means 3 to 6 times more wear and tear on the damper motors and everything else. I'd look closely as to why you are having early damper failures. I don't think I've had a single damper failure on any of the boilers we've installed in the past ten years. We add them to most of the atmospherics we install that are between 300 and 650,000, besides all those on the residential models.

ChrisJ

The Steam Whisperer said:

@Steamhead I agree with everything you stated except the problems persist with the flue dampers. It seems like every time my company installs an atmospheric boiler and leaves the flue damper operational it always results in a service call as a result in 2 or 3 years.  I generally love Field controls products but not those flue dampers. And I'm no fan of the L8148E aquastats that seem to accompany them.

I"ve never had problems with stack dampers failing very often. I think most of them fail due to short cycling because the boiler is oversized, the heat anticipator is set incorrectly or the operating controls (like tekmar) are not set up properly. I've seen plenty of boilers cycling every 10 minutes instead of once or twice an hours... that means 3 to 6 times more wear and tear on the damper motors and everything else. I'd look closely as to why you are having early damper failures. I don't think I've had a single damper failure on any of the boilers we've installed in the past ten years. We add them to most of the atmospherics we install that are between 300 and 650,000, besides all those on the residential models.

Mine is on its 12th year and no signs of issues and I have mine typically cycling twice an hour, 3 times during extreme cold.

But I did move mine up and away from the drafthood so the motor and gearbox isn't roasting.  Whether or not that really matters I have no idea.  

SuperTech

I think my experiences with the flue dampers failing is primarily due to seeing them on hydronic boilers with multiple zones and of course the boilers are oversized most of the time. Lots of cycling and wear and tear. I'm sure they would last longer on a properly sized steam boiler. 

ethicalpaul

OK dampers are good, but they are not what this thread is about

Have I imagined it that people have said that as a firing cycle goes on, the efficiency improves? What do they mean by this?

Thanks!

Chris_L

@ethicalpaul I think you need to consider the different measures of boiler efficiency. See for example, https://www.abma.com/assets/docs/Tech_Resources/2015 - commercial_boiler_efficiency.determine.test_2008.pdf

As I understand it, CO will be higher on startup, and thus the combustion efficiency (measured with a stack test) will be lower. Once the boiler reaches operating temperature, the opposite will be true.

ethicalpaul

Page one of that document has me wondering just how accurate it can be...this is completely wrong



The combustion occurs at the burner, which has no idea if the boiler has reached operating temperature, so that's the nature of my question about this.

EBEBRATT-Ed

what @ChrisJ said. It sometimes takes a while for the boiler to get to steady state. Just like a car needs a air/fuel adjustment like a choke or something in the old days.

ChrisJ

EBEBRATT-Ed said:

what @ChrisJ said. It sometimes takes a while for the boiler to get to steady state. Just like a car needs a air/fuel adjustment like a choke or something in the old days.

A gasoline engine needs a richer fuel mixture because of fuel not atomizing as good as well as a large percentage condensing on cold intake and cold cylinder walls.


Engines running on lpg rarely need such treatment and ones running on natural gas never need it.   My generator has no choke of any kind and starts and runs absolutely normal Immediately.

ethicalpaul

EBEBRATT-Ed said:

what @ChrisJ said. It sometimes takes a while for the boiler to get to steady state. Just like a car needs a air/fuel adjustment like a choke or something in the old days.

Yep, I can get that. After all, the atomized air-fuel mix is in a cylinder and the temperature of that cylinder will affect the combustion.

But a burner sits below the boiler, and is not connected to the temperature of the boiler, right?

ChrisJ

For what it's worth natural gas has a boiling point of -260f so as long as your burner tubes are above that you shouldn't need a richer mixture. 

Steamhead

SuperTech said:

I think my experiences with the flue dampers failing is primarily due to seeing them on hydronic boilers with multiple zones and of course the boilers are oversized most of the time. Lots of cycling and wear and tear. I'm sure they would last longer on a properly sized steam boiler. 

These boilers should have weather-responsive controls so they won't short-cycle so much.

EBEBRATT-Ed

Nothing will combust the same when it is cold as when it is hot. That is just the nature of combustion. If the air is cold or the fuel is cold or the engine, boiler, furnace whatever it is is cold combustion will change when it warms up. Doesn't matter what the fuel is propane, oil, natural gas wood gasoline, diesel etc.

Deisel engines struggle to start when cold. They have a glow plug that is the same temp weather the engine is hot or cold, they have the same compression when hot or cold. Those are the only two things that light them off

Put a pot of cold water on a gas stove burner and it will make CO. Take the pot off and the CO drops to normal

EBEBRATT-Ed

Chris J said "Engines running on lpg rarely need such treatment and ones running on natural gas never need it. My generator has no choke of any kind and starts and runs absolutely normal Immediately."

How true. Natural gas and gasoline doesn't take much to light them off. #6 oil is a whole nother story.

Just because a generator lights off and run perfectly when cold it still will not combustion test the same when hot or cold.

@ethicalpaul

Yes with an atmospheric the burners are under the boiler but it's still like the pot on the gas stove. Combustion will change with the temperature of the environment and or the fuel and combustion air.

This is why older oil burner had brick combustion chambers to reflect heat back into the fire to make them burn better. The brick also protected the boiler from the hot flame.

Not required as much with newer burners that combust better.

Combustion is constantly changing. Run your boiler on a hot day and then on a cold day you will get some difference in combustion #s

On a cold day the air is denser so you get more oxygen. The chimney also pulls more draft on a cold day than a warm day.

hot_rod

I think at least 3 boiler brands now have the ability to adjust constantly adjust combustion, combustion analyzer just used to confirm start up.

LP is a bit of a dirty fuel and temperature can cause issues also. I don’t know how a consumer knows when they start blending NG or LP when resources are low. So the fuel content can change. Another plus for self adjusting ability, I suppose.

The Steam Whisperer

SuperTech said:

I think my experiences with the flue dampers failing is primarily due to seeing them on hydronic boilers with multiple zones and of course the boilers are oversized most of the time. Lots of cycling and wear and tear. I'm sure they would last longer on a properly sized steam boiler. 

That would certainly do it. I'd add an electric water heater to the boiler circuit to give the boiler some mass to work against. Converted gravity systems give great efficiency with the really long burn times... typical winter day is about 30 to 40 minute burner on time with 20 to 30 minutes off. This helps the efficiency of any boiler. Current day gas atmospherics are usually saddled with lanced steel burners running around 50% excess air... the old cast iron burner models usually ran about 20 to 30% excess air for higher flame efficiencies. Stage fired atmospherics properly piped and controlled can run pretty efficient for larger applications. I've seen fuel usage reductions from 20% to 60% with just this upgrade over older boilers.